In a power distribution system, switches (e.g. circuit breakers) are generally relied upon to make or break power supply connections in the system, and in particular to break the power supply connection from a power source to an electrical load. These switches are designed for a rated current, and cut off the current flowing through them in the event of a fault such as a short circuit to ground, i.e. break the power supply connection.
It is generally desired that in the event of a fault, only the power supply connection associated with or closest to the fault is broken; this is referred to as zone selective breaking, or zone selective interlocking (ZSI). Specifically, supposing that a short circuit to ground fault occurs in a multi-stage power distribution network supplied with power by a single source, then each circuit breaker which detects the fault must transmit an interlocking signal (ZSI signal) to the circuit breaker upstream of itself. Thus, only those circuit breakers which do not receive a ZSI signal are determined as being closest to the fault point and execute instantaneous protection (e.g. with a delay time t=50 ms), i.e. perform the breaking action with the shortest time delay. Those circuit breakers which receive a ZSI signal execute timed protection, i.e. are “locked”. By “timed protection” or “locking” is meant that: if the fault is still not eliminated when a predetermined delay time (e.g. 100 ms) expires (e.g. if the protecting action of a circuit breaker closer to the fault point fails), then the circuit breaker will break the circuit, otherwise it will maintain the connection.
The simple ZSI described above is not suitable for a power distribution network with multiple power sources, owing to the increased complexity of the network. Thus, directional zone selective interlocking (DISI) has been proposed. Chinese patent application No. 201310150057.5, submitted on Apr. 26, 2013, has disclosed a method and device for implementing directional zone selective protection. According to the disclosure in that patent application, ZSI signals are transmitted to different circuit breakers selectively according to the direction of current flow through the circuit breaker in question. The advantage of this approach is that, even if there are multiple power sources or active loads in the network, selective protection can be achieved more reliably.
Regardless of whether a ZSI method or DZSI method is used, ZSI signals are transmitted along communication paths between protection devices of the various circuit breakers. Thus, the normal operation of a communication path which transmits ZSI signals becomes a prerequisite for implementing zone selective interlocking.